Off the top of my head, and in my opinion:
If you added highly tactical combat, you wouldn't be playing AD&D anymore.
If you allowed players to purchase magical items with gp as a matter of course (using the magic item appendix as a shopping list), you wouldn't be playing AD&D anymore.
If you altered the character classes to be predicated on combat prowess for the source of their game-play worth/balance, you wouldn't be playing AD&D anymore.
If you disallowed all spontaneous player actions because the proposed actions are not covered explicitly by the written rules, you wouldn't be playing AD&D anymore.
If you change the magic system to something other than the "Vancian" system, you wouldn't be playing AD&D anymore (psionic system notwithstanding).
If you removed all rules and setting elements that inform the humanocentric view, you wouldn't be playing AD&D anymore (setting-wise).
If you allowed "weird" player characters, such as half storm giant/half pixie cross breeds, giants, balrogs, dragons, etc. you wouldn't be playing AD&D anymore. It should be noted that such "weirdness" does fall within the open-ended nature of the original D&D game.